1. Field of the Invention
The present invention relates to a metal member with chromate coat, a method for manufacturing the metal member with chromate coat, a spark plug, and a method for manufacturing the spark plug.
2. Description of the Related Art
The surface of a metal member is often plated for corrosion prevention. For example, in the iron-based material, the surface is often plated with zinc, and it is known that this zinc galvanization is subjected to sacrificial corrosion to present the excellent anti-corrosion. However, there is a drawback that zinc galvanization is consumed severely due to sacrificial corrosion, and changed into white color due to produced zinc oxide, impairing the appearance. Thus, the surface of a zinc plating layer is covered with a chromate coat to prevent the corrosion of the galvanizing layer.
Among the chromate coats, a yellow chromate coat is excellent in corrosion resistance, and is widely used in a broad area including can films, for example. Also, a spark plug used to ignite a gasoline engine in the internal combustion engine of, for example, the automobile, may be often applied with the yellow chromate coat to prevent the corrosion owing to the attack of acid components from inside the engine, or the influence of acid rain during transportation.
However, the yellow chromate coat contains a part of a chromium component in the form of hexavalent chromium, and is shunned increasingly from the respect of environmental problems. Also, there is another drawback that, because a treatment bath of the yellow chromate coat contains a relatively high density of hexavalent chromium, the waste disposal may be expensive. Further, there is another drawback that the yellow chromate coat is inferior in heat resistance, and if the temperature is elevated to about 100xc2x0 C. or higher, the chromate coat is liable to degrade, and the corrosion resistance is rapidly decreased.
On the other hand, the trivalent chromium based chromate coat containing a small amount of hexavalent chromium is excellent in heat resistance, but was often insufficient in its sliding ability when trying to slide with the other member on the surface of a member formed with its chromate coat. For example, in a case that a plastic working which may accompany the sliding with metal such as caulking or deep drawing is applied on a metal member formed with the trivalent chromium based chromate coat, the deformation does not proceed uniformly, unless the sliding is excellently performed, leading to undesired shape or size, or causing a defect such as a crack on the chromate coat. Also, in the case where the chromate coat is formed on a threaded member such as a bolt or nut, it is difficult to rotate a screw smoothly, unless the sliding property is excellent, resulting in either a lower working efficiency or a damaged thread.
It is an object of the present invention to provide a metal member with chromate coat, a method for manufacturing the metal member with chromate coat, a spark plug and a method for manufacturing the spark plug, the chromate coat having a small content of hexavalent chromium, and being excellent in both the corrosion resistance and the sliding property.
In order to accomplish the above object, according to the present invention, there is provided a method for manufacturing a metal member with chromate coat including a chromate treatment step of forming a chromate coat having a film thickness of 0.2 to 0.5 xcexcm and composed of trivalent chromium at 95 mass % or more of chromium component at least on an expected sliding surface by immersing the metal member, having the expected sliding surface that is expected to slide with another member, into a chromating bath, and a heat treatment step of heat treating its chromate coat at temperatures from 85 to 255xc2x0 C.
In the invention, a chromate coat composed of trivalent chromium at 95 mass % or more of chromium component contained and having a film thickness of 0.2 to 0.5 xcexcm (hereinafter referred to as a trivalent chromium based chromate coat) is formed on the surface of the metal member. That is, a normal yellow chromate coat has hexavalent chromium at about 25 to 35 mass % of chromium component, while the chromate coat of this invention contains as small as 5 mass % or less of hexavalent chromium relative to the chromium component, thereby being beneficial for the environment by reducing the hexavalent chromium. Also, a chromate treating solution for use has no content of hexavalent chromium component at all, or a considerably smaller content than the treating solution for the yellow chromate coat, as will be described later. Hence, it is not required to make the reduction of hexavalent chromium, which is needed in the waste disposal of hexavalent chromium, resulting in less drainage problems.
As a result of acute examination, the present inventors have found that if the trivalent chromium based chromate coat is heat-treated in a temperature range from 85 to 255xc2x0 C., the sliding property of trivalent chromium based chromate coat formed on the expected sliding surface of the metal member can be enhanced, whereby this invention has been accomplished.
The trivalent chromium based chromate coat is formed in a wet process, and contains much moisture immediately after forming the film. And if such moisture remains in excessive quantity particularly on the surface of chromate coat, an undesired adsorptive force is developed with the sliding member due to this moisture, and may possibly degrade the sliding ability. Thus, this moisture is subjected to the heat treatment in the above-mentioned temperature range to dry the film adequately, so that the sliding property of the chromate coat can be improved.
By heating the film, the film shrinks and is hardened. Such shrinkage may possibly be caused due to the fact that the hydrated chromate constituting the film is dewatered by heating. Further, as a result of acute examination, it has been found that trivalent chromium based chromate coat shrinks to some extent when a heat history is applied, but is hardened and becomes finer without causing almost any cracks, whereby in sliding, the chromate coat is unlikely to peel from a substrate (e.g., a zinc plating layer), and expose the substrate to give rise to increased sliding friction. Also, the peeling dust that may possibly decrease the sliding ability is unlikely to occur. Consequently, the sliding property can be improved.
At the heat treatment temperatures less than 85xc2x0 C., the sliding property can not be effectively improved. On the other hand, at the heat treatment temperatures above 255xc2x0 C., the sliding property is rather decreased, and the film peeling or lower corrosion resistance may be incurred along with the sliding. This is because if the heat treatment temperature is too high, the chromate coat is excessively hardened and becomes fragile. The heat treatment process may be performed in the air (or atmosphere), or in an inert atmosphere such as a nitrogen gas or Ar gas, for example. The heat treatment temperature is preferably in a range from 90 to 250xc2x0 C.
If the film thickness of the chromate coat is below 0.2 xcexcm, the anti-corrosion performance or heat resistance can not be sufficiently secured. Also, if the film thickness is above 0.5 xcexcm, the film is more likely to crack or fall in sliding. The film thickness of the chromate coat is desirably 0.3 to 0.5 xcexcm. It is desired that the chromate coat does not contain substantially hexavalent chromium.
The metal member can be a metal shell of a spark plug caulked against the outside of an insulator. In this case, when the metal shell is caulked against the insulator, a chromate coat is formed on an outside face of the metal shell containing an expected sliding surface, which is defined as a surface of an expected caulking portion of the metal shell expected to contact and slide with a caulking mold, and after the heat treatment process, the metal shell can be caulked with the expected caulking portion directed toward the insulator.
A method for manufacturing a spark plug according to the invention includes a caulking step of caulking a barrel-like metal shell against an insulator by inserting an axial insulator in an axial direction through an inserting opening portion into the metal shell, compressing a sliding expected surface of a caulking expected portion formed in the opening portion in the axial direction, while contacting and sliding with a caulking mold, and bending the caulking expected portion toward the insulator, a chromate treatment step of forming a chromate coat composed of trivalent chromium at 95 mass % or more of chromium component contained and having a film thickness of 0.2 to 0.5 xcexcm at least on the expected sliding surface by immersing the metal shell in a chromating bath, and a heat treatment step of heat-treating the chromate coat at temperatures from 85 to 255xc2x0 C., wherein the heat treatment step is performed before the caulking step.
When manufacturing the spark plug, if the sliding property between the metal shell and the caulking mold is worse, the chromate coat formed on the metal shell may cause a defect such as a crack, or an undesired force may be applied in caulking to increase the size of a tool engaging portion (a hexagonal portion) for the metal shell. Thus, the chromate coat of the invention is formed on the metal shell of the spark plug, and is subjected to the heat treatment within the scope of the invention. Then if the caulking step of caulking the metal shell against the insulator is made, the sliding property between the metal shell and the caulking mold is enhanced, so that the chromate coat formed on the surface of the metal shell causes less defects such as cracks. And the undesired broadening of the size of the tool engaging portion (so-called hexagonal portion) of the metal shell can be suppressed.
With the above method for manufacturing the metal member with chromate coat according to the invention, the metal member with chromate coat can be produced. That is, the metal member with chromate coat having a sliding face expected to slide or having already slid with the other member (hereinafter generically referred to as a sliding expected surface) is formed with a chromate coat composed of trivalent chromium at 95 mass % or more of chromium component and having a film thickness of 0.2 to 0.5 xcexcm at least on the sliding expected surface, in which the hardness of the sliding expected surface formed with the chromate coat is in a range from 1500 to 3000 N/mm2, in universal hardness (HU), measured under a pressing load of 40 mN for an indenter in accordance with a method as defined in the German standards DIN50359-1.
A spark plug of the invention can be produced by employing a metal member with a chromate coat according to the invention as a metal shell of the spark plug. That is, the spark plug has the metal shell caulked against the outside of an insulator, the metal shell is the metal member, and the expected sliding surface is an outer face of a caulking portion for the metal shell.
By subjecting the chromate coat to the heat treatment as described above, the water content of the chromate coat is removed, so that the chromate coat is made finer and hardened. The present inventors made the measurement of the surface hardness for a metal member with chromate coat having an excellent sliding property with the heat treatment, and found that the chromate coat have a range of hardness suitable to enhance the sliding property. Namely, the metal member with chromate coat having the universal hardness (HU) of the sliding expected surface in a range from 1500 to 3000 N/mm2 under a pressing load of 40 mN at the time of measurement has an excellent sliding property and can smoothly slide with the other member.
The chromate coat formed on the surface of the metal shell, particularly when applied to the spark plug, has less cracks even after caulking, and the broadening of the size of the tool engaging portion can be suppressed. As a result, when a lot comprised of a plurality of spark plugs is produced, it is possible to reduce the variation in the size of the tool engaging portion (e.g., hexagonal portion) formed on an outer peripheral face of each metal shell for the spark plug included within the lot. Thereby, there is less incidence of the spark plugs having the size of the tool engaging portion outside the tolerance, contributing to the improved manufacturing yield.
In the invention, a chromating bath for forming the chromate coat preferably contains a mixture of trivalent chromium salt, and a complexing agent for trivalent chromium. By employing such a chromating bath, a fine and thick trivalent chromium based chromate coat that is difficult to make in accordance with the typical chromate treating method can be formed, and further the chromate coat having a suitable film thickness of 0.2 to 0.5 xcexcm can be easily formed. For such a method for forming the chromate coat, the details were disclosed in German Patent DE19638176A1. The outline of the method will be described below.
According to a theory, in a process for forming the chromate coat, a substrate metal (e.g., zinc) is first oxidized and eluted in a treatment bath, a substrate metal component eluted and a solution containing chromate ions are reacted, so that trivalent chromium forms a polymer complex of a bridge of hydroxyl group or oxygen to settle or sediment in gel on the interface of substrate metal. In this case, for the growth of the chromate coat, the elution of substrate metal and the reaction and sedimentation of the eluted substrate metal and chromate ions must progress in parallel. However, if the chromate coat sediments to some extent, the elution reaction of a substrate metal layer that is a heterogeneous reaction via an interface with the solution is prevented, so that the growth of the chromate coat is retained.
According to the disclosed content of German Patent DE19638176A1, in order to produce a thicker chromate coat, it is important that the elution of substrate metal and the sedimentation of chromate coat due to reaction between the eluted substrate metal component and trivalent chromium are effected at the greater rate, while the rate of reverse dissolution of the chromate coat is made as small as possible. By the above method, an appropriate complexing agent is added into the bath to make trivalent chromium complex to promote the sedimentation of film, and allow the thicker film to be produced.
As the complexing agents, various sorts of chelating agents may be effectively employed, such as dicarboxylic acid, tricarboxylic acid, hydroxy acid, hydroxyl group dicarboxylic acid or hydroxyl group tricarboxylic acid, for example, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimalic acid, cork acid, selenious acid, sebacic acid, maleic acid, phthalic acid, terephthalic acid, tartaric acid, citric acid, malic acid, and ascorbic acid. Other complexing agents may be employed. Usable complexing agents were disclosed in the German Patent described previously.
It is preferable to set the chromating bath at a temperature from 20 to 80xc2x0 C. And the immersing time of the subject in the chromating bath is preferably 20 to 80 seconds. First, if the temperature of the bath is below 20xc2x0 C., the film thickness of the chromate coat formed can not be obtained sufficiently. On the other hand, if the temperature of the bath is 80xc2x0 C. or greater, the evaporation of water content from the bath is so vigorous that the bath condition becomes less accurate. Also, if the immersing time is below 20 seconds, the sufficient chromate coat may not be formed. On the other hand, if the immersing time is beyond 80 seconds, the formed chromate coat becomes too thick, causing a crack on the film, or an exfoliation of the film.
The heat treatment time must be long enough to secure the sliding property in the chromate coat obtained after the heat treatment. The lower limit and the upper limit of the heat treatment time are varied depending on the heat treatment temperature, and the upper limit of the heat treatment time required is shifted to the shorter time side as the heat treatment temperature rises. Specifically, the heat treatment process is performed in a condition satisfying any one of the following conditions,
30xe2x89xa6txe2x89xa61800 for 85xe2x89xa6Txe2x89xa6155
30xe2x89xa6txe2x89xa61500 for 155 less than Txe2x89xa6170
30xe2x89xa6txe2x89xa61200 for 170 less than Txe2x89xa6190
30xe2x89xa6txe2x89xa6900 for 190 less than Txe2x89xa6205
30xe2x89xa6txe2x89xa6600 for 205 less than Txe2x89xa6215
30xe2x89xa6txe2x89xa6480 for 215 less than Txe2x89xa6230
30xe2x89xa6txe2x89xa6300 for 230 less than Txe2x89xa6245
30xe2x89xa6txe2x89xa6240 for 245 less than Txe2x89xa6255
where the heat treatment temperature is T (xc2x0 C.) and the heat treatment time is t (sec).
In any of the conditions, if the heat treatment time is set to be longer than the lower limit, the obtained chromate coat can significantly enhance the sliding property.
The heat treatment process is more preferably performed in a condition satisfying any one of the following conditions,
210xe2x89xa6txe2x89xa61800 for 85 less than Txe2x89xa695
90xe2x89xa6txe2x89xa61800 for 95 less than Txe2x89xa6110
45xe2x89xa6txe2x89xa61800 for 110 less than Txe2x89xa6130
xe2x80x8330xe2x89xa6txe2x89xa61800 for 130 less than Txe2x89xa6145
30xe2x89xa6txe2x89xa61500 for 145 less than Txe2x89xa6155
30xe2x89xa6txe2x89xa61200 for 155 less than Txe2x89xa6170
30xe2x89xa6txe2x89xa6900 for 170 less than Txe2x89xa6190
30xe2x89xa6txe2x89xa6600 for 190 less than Txe2x89xa6205
30xe2x89xa6txe2x89xa6480 for 205 less than Txe2x89xa6215
30xe2x89xa6txe2x89xa6300 for 215 less than Txe2x89xa6230
30xe2x89xa6txe2x89xa6240 for 230 less than Txe2x89xa6245
30xe2x89xa6txe2x89xa6180 for 245 less than Txe2x89xa6255
where the heat treatment temperature is T (xc2x0 C.) and the heat treatment time is t (sec).
From the experiments made by the present inventors, it was found that the heat treatment enhances the sliding property, while the excessive heat treatment degrades the sliding property. And the heat treatment time t (sec) in a permissible range to enhance the sliding property satisfies the following condition:
txe2x89xa60.94xc3x97exp (4201/T)xe2x80x83xe2x80x83(1)
where the heat treatment temperature is T (K). If the heat treatment time t (sec) is set to be shorter than the upper limit, the excessive heat treatment can be prevented, so that the sliding property can be improved. Note that the heat treatment time t (sec) is more desirably set such that
txe2x89xa60.038xc3x97exp (4489/T)xe2x80x83xe2x80x83(2)